CN116781174B - FSO transmitting power self-adaptive system based on reciprocity of atmospheric turbulence channel - Google Patents

Abstract

The invention provides an FSO transmitting power self-adaptive system based on atmosphere turbulence channel reciprocity, which comprises: a first optical terminal Alice and a second optical terminal Bob; the first optical terminal is used for a transmitting end of a communication system, generating a gain adjustable signal and realizing self-adaptive power control of optical power; the second optical terminal is used for a receiving end of the communication system. The invention takes delay time and system noise into consideration, derives a reciprocity evaluation model of Gamma-Gamma continuous time signals by utilizing a Yule-Worker equation in an ARMA random process, and provides a power self-adaptive control algorithm based on reciprocity, thereby effectively relieving receiving end light intensity flicker and communication error rate deterioration caused by atmospheric turbulence.

Description

FSO Transmit Power Adaptive System Based on Reciprocity of Atmospheric Turbulence Channel

Technical Field

The invention belongs to the technical field of laser communication, and in particular relates to an FSO transmission power adaptive system based on atmospheric turbulence channel reciprocity.

Background Art

With the rise of intelligent interactive networks, fiber optic technology is managing most of the ground access and backbone networks with a total capacity of tens of terabits per second (Tbps). In order to solve the high cost of laying fiber optic networks in remote areas, space laser communication (FSOC) technology can be used to fill this gap through satellites and high-altitude platforms, especially to provide backhaul service coverage to sparsely populated areas. However, the disturbance of atmospheric turbulence seriously destroys the wavefront of the propagating beam, which limits the development of FSOC technology. New methods are needed to suppress the increase in bit error rate caused by turbulent disturbances.

For the adaptive processing system at the receiving end, it is usually necessary to use a large number of training sequences at the receiving end to obtain channel state information (CSI) for signal compensation processing. These operations need to be completed within the coherence time of turbulent disturbances. Due to the existence of training sequences, real-time processing cannot be achieved, resulting in high system hardware overhead and high latency, which seriously restricts the real-time and high-speed performance of laser communication. For example, the AO system designed by A.V.Kudryashov et al. needs to obtain detailed wavefront information of the light spot at the receiving end and then feedback to adjust the deformable reflector, and the control system is complex.

Using the reciprocity of the atmospheric turbulence channel to obtain CSI can greatly improve this problem. However, studies have shown that FSOC is not absolutely reciprocal, and the conditions for maintaining reciprocity are strict. In practice, we cannot perfectly keep the system reciprocity of FSOC at 1. Most radio channels are in a reciprocity mismatch state, which makes it impossible for us to achieve the best performance by using reciprocity for adaptive control of the transmitting optical power. Therefore, it is necessary to further study and analyze the relationship between the reciprocity of the atmospheric turbulence system and the working principle and performance of the FSO transmit power adaptive system.

Summary of the invention

In order to solve the above technical problems, the present invention proposes an FSO transmission power adaptive system based on the reciprocity of atmospheric turbulence channels, so that the light intensity flicker of the communication signal can be well suppressed, the signal-to-noise ratio is greatly improved, the suppression is more obvious under strong turbulence, and the flicker suppression is -3.5dB.

To achieve the above object, the present invention proposes an FSO transmission power adaptation system based on atmospheric turbulence channel reciprocity, comprising: a first optical terminal Alice and a second optical terminal Bob;

The first optical terminal is used at the transmitting end of the communication system to generate a gain-adjustable signal to achieve adaptive power control of optical power;

The second optical terminal is used at the receiving end of the communication system.

Optionally, the first optical terminal and the second optical terminal have the same structure;

The first optical terminal and the second optical terminal both include: an optical antenna, a dense wavelength division multiplexing system DWDM, a detection unit, an automatic gain feedback adjustment unit and a transmitting unit connected in sequence;

The optical antenna is used to transmit and receive spatial optical communication signals;

The dense wavelength division multiplexing system is used to multiplex light waves with wavelengths of λ ₁ -λ ₄ ;

The transmitting unit is used to transmit light waves with wavelengths of λ ₁ -λ ₄ ;

The automatic gain feedback adjustment unit is used to automatically adjust the transmitted optical power to suppress the influence of turbulence;

The detection unit is used to convert light waves with wavelengths of λ ₁ -λ ₄ into electrical signals.

Optionally, the transmitting unit includes: a laser, an arbitrary waveform generator, a microwave signal amplifier, a lithium niobate modulator and an erbium-doped fiber amplifier; the laser is connected to the optical input end of the lithium niobate modulator; the arbitrary waveform generator, the microwave signal amplifier, the lithium niobate modulator and the erbium-doped fiber amplifier are connected in sequence, and the microwave signal amplifier is also connected to the electrical signal input end of the lithium niobate modulator;

The laser is used to generate light waves of λ ₁ -λ ₄ ;

The arbitrary waveform generator is used to generate a continuous baseband signal through a raised cosine filter;

The microwave signal amplifier is used to amplify the baseband signal;

The lithium niobate modulator is used to load the amplified baseband signal onto the optical wave to generate a continuous optical baseband signal and output a modulated communication optical signal;

The erbium-doped fiber amplifier is used to amplify the communication optical signal.

Optionally, the automatic gain feedback adjustment unit includes: an attenuator and an FPGA;

The attenuator is used to receive the amplified communication optical signal and attenuate the communication optical signal to generate a transmission optical signal;

The FPGA is used to control the attenuation size of the attenuator.

Optionally, the detection unit includes: a dense wavelength division multiplexing system DWDM, a high-speed detector, a low-speed detector and a digital storage oscilloscope; wherein, the dense wavelength division multiplexing system has a λ ₂ wavelength output, and the high-speed detector is connected to the digital storage oscilloscope in sequence; the dense wavelength division multiplexing system has a λ ₄ wavelength output, and the low-speed detector is connected to the FPGA in sequence.

The dense wavelength division multiplexing system is used to multiplex light waves with wavelengths of λ ₂ and λ ₄ ;

The high-speed detector is used to detect high-speed communication optical signals and convert them into electrical signals for output;

The low-speed detector is used to detect the fading characteristics of the turbulence signal and obtain it by processing in the FPGA;

The digital storage oscilloscope is used to store the acquisition results.

Optionally, the transmission power adaptation process of the first optical terminal and the second optical terminal is:

in, is the convolution process, g _A (t) is the matched filter, ρ _A is the exponential of EDFA amplification at Alice’s end, n _A (t) is the additive white noise at Alice’s end, Generates gain-adjustable signals for FPGA, To transmit optical signals, Extracting optical signals for reciprocity feature information.

Optionally, the FPGA generates a gain adjustable signal include:

The low-speed detector detects the jitter light wave signal generated by Bob→Alice passing through the turbulent channel The signal is received and sampled by the detector to obtain the power spectrum function

The power spectrum function As shown in the following formula (3):

Where: The signal's autocovariance function ACF digital signal form;

a _i is the coefficient of the filter of the transfer function of the atmospheric turbulence channel;

Power spectrum function Solve to obtain a sequence with time correlation; {I ₁ ,I ₂ ,…In}=I, as shown in equations (4) to (6):

σ ² _n H(ω)＝I _n (6)

Where:

Gamma-Gamma (GG) is used as the probability density function PDF of the turbulent channel, and the parameters α and β of the probability density function PDF and the turbulence parameters When linked together, the form is as follows:

Where: express The nth moment of the signal, Γ(·) is the Gamma function;

Let the normalized random light intensity sequence Obey Gamma-Gamma PDF, and make After rearranging in order of I, we get And meet:

The above formula generates of is equal to the numerical solution R _ψ (m) of the theoretical solution, and does not lose PDF information; similarly, according to equations (3) to (10), we can obtain Digital sequence signal

Reciprocity can be used to obtain the channel state information of the Alice→Bob beam propagation process in real time, that is, there is:

η _AB ＝1 (11)

Where η _AB is the reciprocity value of the channel, Extract optical signals for the reciprocity feature information of Alice and Bob respectively, The gain is adjustable.

Compared with the prior art, the present invention has the following advantages and technical effects:

The present invention takes into account the delay time and system noise, derives the reciprocity evaluation model of the Gamma-Gamma continuous-time signal by using the Yule-Worker equation in the ARMA random process, and proposes a power adaptive control algorithm based on reciprocity, which effectively alleviates the light intensity flicker at the receiving end and the deterioration of the communication bit error rate caused by atmospheric turbulence.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings constituting a part of the present application are used to provide a further understanding of the present application. The illustrative embodiments and descriptions of the present application are used to explain the present application and do not constitute an improper limitation on the present application. In the drawings:

FIG1 is a block diagram of an FSO transmission power adaptation system based on atmospheric turbulence channel reciprocity according to an embodiment of the present invention; wherein (a) is a first optical terminal Alice, (b) is an atmospheric turbulence channel, and (c) is a second optical terminal Bob;

FIG2 is a PDF comparison diagram of Alice and Bob reciprocal optical signal sequences before and after automatic gain calibration under different turbulence levels according to an embodiment of the present invention; wherein (a) is the result under strong turbulence, (b) is the result under medium turbulence, and (c) is the result under weak turbulence;

FIG3 is a BER comparison diagram of Alice and Bob reciprocal optical signal sequences before and after automatic gain calibration under different signal-to-noise ratios according to an embodiment of the present invention; wherein (a) is the result under strong turbulence, (b) is the result under medium turbulence, and (c) is the result under weak turbulence;

FIG4 is a BER comparison diagram of Alice and Bob reciprocal optical signal sequences before and after automatic gain calibration at different sampling delays according to an embodiment of the present invention.

DETAILED DESCRIPTION

It should be noted that, in the absence of conflict, the embodiments and features in the embodiments of the present application can be combined with each other. The present application will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

It should be noted that the steps shown in the flowcharts of the accompanying drawings can be executed in a computer system such as a set of computer executable instructions, and that, although a logical order is shown in the flowcharts, in some cases, the steps shown or described can be executed in an order different from that shown here.

As shown in Fig. 1, this embodiment proposes an FSO transmission power adaptation system based on atmospheric turbulence channel reciprocity, which includes two optical terminals, a first optical terminal and a second optical terminal, denoted as Alice and Bob. Fig. 1 (a) is the first optical terminal Alice, Fig. 1 (b) is the atmospheric turbulence channel, and Fig. 1 (c) is the second optical terminal Bob.

The optical terminal consists of an optical antenna, a dense wavelength division multiplexing system DWDM, a transmitting unit, an automatic gain feedback adjustment unit, and a detection unit. The transmitting unit includes a laser LD1, an arbitrary waveform generator (AWG), a microwave signal amplifier (MPA), a lithium niobate modulator (LNM), and an erbium-doped fiber amplifier (EDFA). The automatic gain feedback adjustment unit includes an electrically adjustable optical attenuator (EVOA) and an FPGA. The detection unit includes a fiber splitter (FOS), a high-speed detector, a low-speed detector, and a digital storage oscilloscope (DSO).

The optical terminals Alice and Bob have the same structure and the same transmission power adaptive control method.

For example, Alice's transmitted light wave The generation of includes the following steps:

Step 1, laser LD1 at Alice's end generates a light wave with a wavelength of λ ₁ ;

Step 2, the arbitrary waveform generator generates a continuous baseband signal s _AM (t) through a raised cosine filter and amplifies it through a microwave amplifier;

Step 3: The lithium niobate modulator loads the modulation signal onto the above-mentioned light wave λ ₁ to generate a continuous optical baseband signal.

Step 4: The optical baseband signal is injected into the electrically controlled optical attenuator EVOA after passing through the EDFA to generate a transmitted optical signal.

This process can be expressed using the following mathematical physics equation:

Where: is the convolution process, g _A (t) is the matched filter, ρ _A is the exponential of EDFA amplification at Alice’s end, n _A (t) is the additive white noise at Alice’s end, Generates gain-adjustable signals for FPGA, To transmit optical signals, Extracting optical signals for reciprocity feature information.

It is a gain-varying continuous feedback signal with low latency in practical engineering. In this system, the turbulence signal can be detected by a low-speed photoelectric detector. The fading characteristics are obtained by FPGA processing.

EVOA can accurately control the attenuation size by FPGA, and its automatic gain factor The acquisition method is as follows:

First, the low-speed detector (PD) detects the jittered light wave signal generated by Bob→Alice passing through the turbulent channel. The signal is received and sampled by the detector in the form of a digital sequence set Its power spectrum function As shown below:

Where: for The signal's autocovariance function ACF is in digital signal form.

a _i is the coefficient of the filter of the transfer function of the atmospheric turbulence channel.

The autoregressive moving average process can be used to solve equation (3), so there is:

By using the recursive method of the Yule-Walker equation to solve equations (4) and (5), we can obtain the time-correlated sequence {I ₁ ,I ₂ ,…I _n }=I, as shown in equations (6) to (8):

σ ² _n H(ω)＝I _n (8)

Where: Gamma-Gamma (GG) is used as the probability density function PDF of the turbulent channel. The parameters α and β of this function can be related to the turbulence parameters When linked together, the form is as follows:

Where: express The nth moment of the signal, Γ(·) is the Gamma function.

Furthermore, let the normalized random light intensity sequence Obey Gamma-Gamma PDF, and make After rearranging in order of I, we get And meet:

Generate at this time of is equal to the numerical solution R _ψ (m) of the theoretical solution, and does not lose PDF information. Similarly, according to equations (3) to (12), we can obtain Digital sequence signal

To calibrate the signal at Bob's end Theoretically, the jitter caused by passing through the atmospheric turbulence channel exists:

Reciprocity can be used to obtain the channel state information of the Alice→Bob beam propagation process in real time, that is, there is:

η _AB ＝1 (14)

Therefore, according to the signal Can be calculated in FPGA Automatic gain factor, thereby using reciprocity to accurately control the EVOA modulation attenuation.

Therefore, we can collect the light wave signal from Bob to Alice. Using reciprocity, we can obtain the channel state from Alice to Bob in real time, and then obtain The automatic gain factor completes the automatic gain adjustment of the transmit power as shown in formula (1), thereby suppressing the signal flicker caused by turbulent disturbance and reducing the BER.

In order to keep the attenuation characteristics of the light waves consistent, the light wavelengths λ ₁ -λ ₄ are four adjacent wavelengths in the C band.

For example, for the reciprocity-based power adaptive control method, the time domain signals of the real states of continuous QAM-16 and QAM-32 are integrated with the Gamma-Gamma reciprocal turbulence continuous signal model, and the PDF and BER performance after power adaptive control correction are analyzed.

For high-speed signals, the bit error rate is obtained by threshold judgment of the received signal and long-term accumulation statistics. Therefore, under the equivalent signal-to-noise ratio, the equivalent low-speed signal can be used as the experimental sample for analysis. Considering that the frequency of turbulent disturbance is less than 100Hz, in order to more easily capture the bit error rate and increase the bit error rate resolution, the bit error statistics of the information transmitted from Alice to Bob in 1 second are calculated. The number of samples is 1000, the number of code elements in the continuous signal in 1 second is set to 1.0×10 ⁶ bits, the upsampling frequency is set to 4 times the code element rate, the rolling coefficient of the raised cosine filter is set to 0.025, and the received optical signal is mean-normalized to I/<I>.

As shown in Figure 2, (a) of Figure 2 is the result under strong turbulence, (b) of Figure 2 is the result under medium turbulence, and (c) of Figure 2 is the result under weak turbulence; when SNR = 25dB, the PDF and constellation diagram of the Alice→Bob signal under different turbulence levels. As the turbulence level increases, the uncorrected received optical signal The PDF peak of The peak-to-peak value of the PDF of the signal tends to be closer to "1" as the turbulence level decreases, indicating that the corrected The envelope signal should theoretically be a straight line with stable power and no flickering of light intensity.

In addition, as shown in the constellation diagram, using After automatic gain correction, the error degradation is greatly improved.

As shown in FIG3 , FIG3 (a) is the result under strong turbulence, FIG3 (b) is the result under medium turbulence, and FIG3 (c) is the result under weak turbulence. After automatic gain correction, the BER degradation has been greatly reduced. The stronger the turbulence, the more obvious the correction effect. It can be seen that all BER curves of QAM-32 signals deviate to the right, indicating that under the same conditions, the bit error rate of QAM-16 is superior to that of QAM-32.

As shown in Figure 4, when SNR = 50dB, the system BER is lower than 3.8× ^10-3 of FEC, which requires strong turbulence. τ _d ≤0.186ms, medium turbulence τ _d ≤1.129ms, and weak turbulence τ _d ≤9.028ms. From the trend of the BER curve shown in Figure 4, this just meets the current 5G and 6G system requirements for system delay control at the ms level.

It should be noted that the processor in the embodiment of the present application is not limited to FPGA, and can be an integrated circuit chip with signal processing capabilities. In the implementation process, each step of the above method embodiment can be completed by an integrated logic circuit of hardware in the processor or an instruction in the form of software. The above processor can be a general-purpose processor, a digital signal processor (DSP), an application-specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic devices, discrete gates or transistor logic devices, discrete hardware components. The methods, steps and logic block diagrams disclosed in the embodiments of the present application can be implemented or executed. The general-purpose processor can be a microprocessor or the processor can also be any conventional processor, etc. The steps of the method disclosed in the embodiment of the present application can be directly embodied as a hardware decoding processor to perform, or the hardware and software modules in the decoding processor can be combined and performed. The software module can be located in a mature storage medium in the field such as a random access memory, a flash memory, a read-only memory, a programmable read-only memory or an electrically erasable programmable memory, a register, etc. The storage medium is located in a memory, and the processor reads the information in the memory and completes the steps of the above method in combination with its hardware.

The present invention takes into account the delay time and system noise, derives the reciprocity evaluation model of the Gamma-Gamma continuous-time signal by using the Yule-Worker equation in the ARMA random process, and proposes a power adaptive control algorithm based on reciprocity, which effectively alleviates the light intensity flicker at the receiving end and the deterioration of the communication bit error rate caused by atmospheric turbulence.

The above are only preferred specific implementations of the present application, but the protection scope of the present application is not limited thereto. Any changes or substitutions that can be easily thought of by any technician familiar with the technical field within the technical scope disclosed in the present application should be included in the protection scope of the present application. Therefore, the protection scope of the present application should be based on the protection scope of the claims.

Claims (5)

1. An FSO transmit power adaptive system based on atmospheric turbulence channel reciprocity, comprising: a first optical terminal Alice and a second optical terminal Bob;

The first optical terminal is used for a transmitting end of a communication system, generating a gain adjustable signal and realizing self-adaptive power control of optical power;

the second optical terminal is used for a receiving end of the communication system;

the self-adaptive process of the transmitting power of the first optical terminal and the second optical terminal is as follows:

wherein, For the convolution process, g _A (t) is a matched filter, ρ _A is an index of Alice end EDFA amplification, n _A (t) is Alice end additive white noise,A gain-adjustable signal is generated for the FPGA,In order to emit an optical signal,Extracting an optical signal for the reciprocity characteristic information;

the FPGA generates a gain adjustable signal Comprising the following steps:

the low-speed detector detects reciprocity characteristic information of Alice end, which is generated by jitter generated by Bob- & gt Alice through turbulent flow channel, and extracts optical signals Reciprocity characteristic information extraction optical signalIs received and sampled by a detector to obtain a power spectrum function

The power spectrum functionThe following formula (3):

Wherein: Extracting optical signals for reciprocity feature information An auto-covariance function ACF digital signal form;

a _i is the constitution coefficient of the filter of the transfer function of the atmospheric turbulence channel;

For power spectrum functions Solving to obtain a sequence with time correlation; { I ₁,I₂,…I_n } = I, as shown in formula (6) -formula (8):

σ² _nθ_n＝I_n(6)

σ² _nH(ω)＝I_n(8)

Wherein:

Gamma-Gamma (G-G) is used as a probability density function PDF of a turbulence channel, and parameters alpha and beta of the probability density function PDF and a turbulence parameter C _n ² are associated, wherein the form is as follows:

Wherein: extracting optical signals representing reciprocity characteristic information Is selected from the group consisting of the nth moment, Γ (·) is a Gamma function;

let normalized random light intensity sequence Obeying the Gamma-Gamma PDF and makingIs rearranged according to order I to obtainAnd satisfies the following:

description generation above A kind of electronic deviceA numerical solution R _ψ (m) equal to the theoretical solution, and no PDF information is lost; similarly, the reciprocity characteristic information of the Bob end is obtained according to the ideas of the formulas (3) and (6) -12 to extract the digital sequence signal of the optical signal

The reciprocity is utilized to acquire the channel state information of Alice- & gt Bob light beam propagation process in real time, namely the channel state information exists:

η_AB＝1 (14)

where η _AB is the reciprocity value of the channel, Extracting optical signals for the reciprocity characteristic information of the Alice end and the Bob end respectively,Is a gain adjustable signal;

And further extracting the optical signal by collecting the reciprocity characteristic information of Bob-Alice Acquiring channel states of Alice- & gtBob in real time by utilizing reciprocity, and further obtainingAnd (3) an automatic gain factor, and completing automatic gain adjustment of the transmitting power as shown in the formula (1).

2. The FSO transmit power adaptation system based on atmospheric turbulence channel reciprocity according to claim 1, wherein the first and second optical terminals are identical in structure;

the first optical terminal and the second optical terminal each include: the system comprises an optical antenna, a dense wavelength division multiplexing system DWDM, a detection unit, an automatic gain feedback adjusting unit and a transmitting unit which are connected in sequence;

the optical antenna is used for transmitting and receiving space optical communication signals;

the dense wavelength division multiplexing system is used for multiplexing the light waves with the wavelength lambda ₁-λ₄;

the emitting unit is used for emitting light waves with the wavelength of lambda ₁-λ₄;

the automatic gain feedback adjusting unit is used for automatically adjusting the emitted light power to inhibit turbulence influence;

the detection unit is used for converting the light wave with the wavelength lambda ₁-λ₄ into an electric signal.

3. The FSO transmit power adaptation system based on atmospheric turbulence channel reciprocity according to claim 2, wherein the transmit unit comprises: the device comprises a laser, an arbitrary waveform generator, a microwave signal amplifier, a lithium niobate modulator and an erbium-doped fiber amplifier; the laser is connected with the light input end of the lithium niobate modulator; the arbitrary waveform generator, the microwave signal amplifier, the lithium niobate modulator and the erbium-doped fiber amplifier are connected in sequence, the microwave signal amplifier is also connected to the electric signal input end of the lithium niobate modulator;

the laser is used for generating light waves of lambda ₁-λ₄;

The arbitrary waveform generator is used for generating a continuous baseband signal through a raised cosine filter;

the microwave signal amplifier is used for amplifying the baseband signal;

The lithium niobate modulator is used for loading the amplified baseband signal onto the light wave to generate a continuous optical baseband signal and outputting a modulated communication optical signal;

The erbium-doped fiber amplifier is used for amplifying the communication optical signal.

4. The FSO transmit power adaptation system based on atmospheric turbulence channel reciprocity according to claim 3, wherein the automatic gain feedback adjustment unit comprises: an attenuator and an FPGA;

The attenuator is used for receiving the amplified communication optical signal and attenuating the communication optical signal to generate a transmitting optical signal;

the FPGA is used for controlling the attenuation of the attenuator.

5. The FSO transmit power adaptation system based on atmospheric turbulence channel reciprocity according to claim 4, wherein the detection unit comprises: dense wavelength division multiplexing system DWDM, high-speed detector, low-speed detector and digital storage oscilloscope; wherein, the wavelength output of the dense wavelength division multiplexing system lambda ₂, the high-speed detector is sequentially connected with the digital storage oscilloscope; the wavelength output of the dense wavelength division multiplexing system lambda ₄ is carried out, and the low-speed detector is sequentially connected with the FPGA;

the dense wavelength division multiplexing system is used for multiplexing the light waves with the wavelength lambda ₂,λ₄;

The high-speed detector is used for detecting high-speed communication optical signals and converting the high-speed communication optical signals into electric signals to be output;

The low-speed detector is used for detecting the fading characteristic of the turbulence signal and is obtained through processing by the FPGA;

the digital storage oscilloscope is used for storing the acquisition result.